Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  Heqcall : ([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    let
    '(prefix, l1', l2') := split_prefix (a1 :: l1) (a2 :: l2) in
     a1 :: l1 = prefix ++ l1' ∧ a2 :: l2 = prefix ++ l2')


DEBUG:Going to execute:
fail 1